The general procedure utilized in the preparation of extruded synthetic foam bodies generally includes the steps of melting a base polymeric composition, incorporating one or more blowing agents and other additives into the polymeric melt under conditions that provide for the thorough mixing of the blowing agent and the polymer while preventing the mixture from foaming prematurely, e.g., under pressure. This mixture is then typically extruded through a single or multi-stage extrusion die to cool and reduce the pressure on the mixture, allowing the mixture to foam and produce a foamed product. As will be appreciated, the relative quantities of the polymer(s), blowing agent(s) and additives, the temperature and the manner in which the pressure is reduced will tend to affect the qualities and properties of the resulting foam product. As will also be appreciated, the foamable mixture is maintained under a relatively high pressure until it passes through an extrusion die and is allowed to expand in a region of reduced pressure. Although reduced relative to the pressure at the extrusion die, the reduced pressure region may actually be maintained at a pressure above atmospheric pressure, for example up to about 2 atm or even more in some applications, may be maintained at a pressure below atmospheric pressure, for example down to about 0.25 atm or even less in some applications. Further, unless indicated otherwise, all references to pressure provided herein are stated as the absolute pressure.
The solubility of conventional blowing agents, such as chlorofluorocarbons (“CFCs”) and certain alkanes, in polystyrene tends to reduce the extrusion melt viscosity and improve cooling of expanded polystyrene melts. For example, the combination of pentane and a CFCs such as Freon 11 and 12 is partially soluble in polystyrene and has been used for generating polystyrene foams that exhibited a generally acceptable appearance and physical properties such as surface finish, cell size and distribution, orientation, shrinkage and stiffness.
However, in response to the apparent contribution of such CFC compounds to the reduction of the ozone layer in Earth's stratosphere, the widespread use and accompanying atmospheric release of such compounds in applications such as aerosol propellants, refrigerants, foam-blowing agents and specialty solvents has recently been drastically reduced or eliminated by government regulation.
The divergence away from the use of CFCs has led to utilization of alternative blowing agents, such as hydrogen-containing chlorofluoroalkanes (HCFCs). However, while HCFC's are considered to be environmentally friendly blowing agents compared to CFCs, such compounds do still contain some chlorine and are therefore said to have an ozone depletion potential.
Another alternative class of blowing agents, hydrofluorocarbons (HFC's), are now being commonly used as more ozone friendly options. Particularly, CF3CH2CF2H (HFC-245fa), 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a), offer desirable improvements, such as improved insulation, due at least in part to the low thermal conductivity of the vapor.
Hydrocarbons such as pentane, hexane, cyclopentane and other homologs of this series have also been considered.
A new generation of fluroralkene blowing agents have been developed with low ODP (ozone depletion potential) and low GWP (global warming potential) known as hydroflouroolefins (HFOs). HFOs have been identified as potential low global warming potential blowing agents for the production of thermoplastic foams, such as polystyrene foam, for thermal insulation.